Prevent flickering/endless loops between NetworkSelectionModels.

If either endpoint changes the selection and the remote end acts
upon it, we must never sent any data back to the initial endpoint.
This would otherwise trigger endless loops/ping-pongs.

It shows btw. that the issue lies in lacking support for extending
of selections, which can be done by selection multiple items in a
list by presseing with the mouse and then hovering over other items.
Fix NetworkSelectionModel to properly deselect/select items.

Previously only the last item added to the selection was remoted,
and the existing selection was cleared. Instead we must propagate
what was selected/deselected and add/remove that from the current
selection.

This finally fixes the selection via mouse in e.g. Text Codecs tool.